The present invention relates to an apparatus and method for separation and purification of unsaturated hydrocarbons, more particularly relates to an apparatus and method for separation and purification of unsaturated hydrocarbons which enable a target unsaturated hydrocarbon to be taken out stably at a predetermined concentration.
1,3-butadiene, isoprene, and other conjugated dienes are generally separated and purified as unsaturated hydrocarbons by extractive distillation using an extraction solvent from a C4 fraction or C5 fraction obtained by cracking naphtha and separating the ethylene, propylene, and other C2 and C3 hydrocarbons (Japanese Examined Patent Publication (Kokoku) No. 45-17405, Japanese Examined Patent Publication (Kokoku) No. 45-17411, Japanese Examined Patent Publication (Kokoku) No. 47-41323, Japanese Unexamined Patent Publication (Kokai) No. 56-83421, etc.)
Normally, this extractive distillation is performed using an apparatus comprised of an extractive distillation tower and stripping tower. Conjugated dienes, which dissolve relatively easily in the solvents included in the C4 fraction or C5 fraction, are taken out as mixtures with the solvents from the bottom of the extractive distillation tower and sent to the stripping tower, where the conjugated dienes and solvents are separated. The solvents are then returned to the extractive distillation tower.
In the conventional separation and purification apparatus and method for conjugated dienes, the general practice has been to control the ratio of the extraction solvent fed to the extractive distillation tower, control the flow rate of part of the residual components of the feedstock taken out from the top of the extractive distillation tower (residuum of feedstock after conjugated dienes have been extracted) and reflux it to the extractive distillation tower, control the bottom temperature of the extractive distillation tower, etc. to separate and purify a stable quality of conjugated dienes.
With such a conventional separation and purification apparatus and method of conjugated dienes, however, when the composition of the feedstock fed to the extractive distillation tower varied, the concentration of the target conjugated dienes taken out from the tower varied. Consequently, it was difficult to take out a stable quality of conjugated dienes.
Note that to take out an extract of a high concentration and constant concentration of conjugated dienes from the extractive distillation tower, it is preferable to return the extract taken out from the bottom of the extractive distillation tower to the extractive distillation tower and control the return ratio. If the return ratio to the extractive distillation tower, however, is not allowed to fluctuate in accordance with the ratio of the extraction solvent, the bottom temperature, the bottom pressure, the ratio of the feedstock fed, the concentration of the conjugated dienes in the feedstock, etc., it is not possible to maintain a constant concentration of the target butadiene, isoprene, or other conjugated dienes and concentration of other specific impurities in the extractive distillation tower. Further, it is close to impossible for an operator to manually handle this control procedure. Therefore, at the present time, priority is given to ease of operation even if allowing the concentration of the conjugated dienes taken out from the extractive distillation tower to fluctuate somewhat. The return ratio is not controlled, but the flow rate to the next process is controlled and the surplus is returned. Therefore, there was a large fluctuation in the conjugated dienes taken out from the extractive distillation tower. In particular, if there is a large fluctuation in concentration of the conjugated dienes taken out in the first extractive distillation tower used for the separation and purification apparatus of the conjugated dienes, increasing the purity of the conjugated dienes in the subsequent processes becomes difficult and stably obtaining high purity conjugated dienes becomes difficult.
An object of the present invention is to provide an apparatus and method for separation and purification of unsaturated hydrocarbons which enable a target conjugated diene or other unsaturated hydrocarbons to be stably taken out at a predetermined concentration regardless of variations in the components of the feedstock or fluctuations in the ratio of the feedstock fed.
To achieve this object, the apparatus for separation and purification apparatus of an unsaturated hydrocarbon of the present invention comprises an extractive distillation tower fed with feedstock containing unsaturated hydrocarbons and a solvent and distilling the feedstock to separate and purify a target unsaturated hydrocarbon; an impurity concentration detecting means for detecting a concentration of a specific impurity other than the target unsaturated hydrocarbon at the extractive distillation tower or another tower connected to the extractive distillation tower (the extractive distillation tower, etc.); a target material concentration detecting means for detecting a concentration of the target unsaturated hydrocarbon at the extractive distillation tower or another tower connected to the extractive distillation tower; a return ratio control means for controlling a flow rate of part of a fluid including the target unsaturated hydrocarbon taken out from a bottom of the extractive distillation tower and returned to the extractive distillation tower; a solvent ratio control means for controlling a ratio of the solvent fed to the extractive distillation tower; a reflux ratio control means for controlling a flow rate of part of a residual component of the feedstock taken out from a top of the extractive distillation tower (meaning the residuum of the feedstock after the unsaturated hydrocarbons has been extracted, but also including some unsaturated hydrocarbons, same below) and refluxed to the extractive distillation tower; a bottom temperature control means for controlling a bottom temperature of the extractive distillation tower; and a predictive control means for calculating forecasted values of the concentration of the specific impurity and the concentration of the target unsaturated hydrocarbon after a predetermined time based on values detected by the impurity concentration detecting means and target material concentration detecting means and controlling the return ratio control means, solvent ratio control means, reflux ratio control means, and bottom temperature control means based on the forecasted values.
Further, the method for separation and purification of an unsaturated hydrocarbon according to the present invention comprises the steps of feeding a feedstock containing a target unsaturated hydrocarbon and a solvent to an extractive distillation tower; detecting a concentration of a specific impurity other than the target unsaturated hydrocarbon at the extractive distillation tower or another tower connected to the extractive distillation tower; detecting a concentration of the target unsaturated hydrocarbon at the extractive distillation tower or another tower connected to the extractive distillation tower; controlling a return flow rate of part of a fluid containing the target unsaturated hydrocarbon taken out from a bottom of the extractive distillation tower and returned to the extractive distillation tower; controlling a ratio of the solvent fed to the extractive distillation tower; controlling a reflux flow rate of part of a residual component of the feedstock taken out from a top of the extractive distillation tower and refluxed to the extractive distillation tower; controlling a bottom temperature of the extractive distillation tower; and calculating forecasted values of the concentration of the specific impurity and the concentration of the target unsaturated hydrocarbon after a predetermined time based on values detected by the impurity concentration detecting step and target material concentration detecting step and controlling the return flow rate, the ratio of the solvent, the reflux flow rate, and the bottom temperature based on the forecasted values.
The solvent used in the present invention may be dimethylformamide, diethylformamide, dimethylacetomide, and other N-alkyl substituted lower fatty acid amides, furfural, N-methylpyrrolidone, formylmorpholine, xcex2-methoxypropionitrile, and other solvents used for extractive distillation of diolefins from hydrocarbon fractions for example. These solvents may be used alone or may be used in mixtures of two or more types. Further, to adjust the boiling point, suitable amounts of water, methanol, etc. may be mixed. Further, it is also possible to jointly use polymerization inhibitors to inhibit polymerization of the diolefins and acetylenes, antioxidants, defoaming agents, etc.
The extraction medium (solvent) is preferably fed to the extractive distillation tower from an extractive distillation medium feed means provided at a position higher than the position feeding the petroleum fraction containing the unsaturated hydrocarbons in the extractive distillation tower (petroleum fraction feed means).
Further, the polymerization inhibitor may be continuously fed from a position higher than the feed position of the extraction medium. As the position higher than the extraction medium feed position, for example, mention may be made of the side of the extractive distillation tower higher than the extraction medium feed position or the inlet or outlet of the condenser of the top of the extractive distillation tower. Among these, provision at the inlet of the top condenser is preferable in that it enables the production of polymers inside the condenser to be suppressed and enables the production of polymers even in processes after the separator to be suppressed. The polymerization inhibitor is preferably one which stops or suppresses polymerization by a chain transfer reaction, in particular, a lower alkylhydroxylamine.
The feedstock (petroleum fraction) used in the present invention contains unsaturated hydrocarbons. The petroleum fraction normally is obtained by cracking naphtha. As the petroleum fraction, there are for example a C2 fraction containing mainly C2 hydrocarbons, a C3 fraction containing mainly C3 hydrocarbons, a C4 fraction containing mainly C4 hydrocarbons, and a C5 fraction containing mainly C5 hydrocarbons. Among these, a fraction increased in the concentration of the unsaturated hydrocarbons due to the extractive distillation etc. is preferred. Further, a fraction containing a large amount of conjugated dienes as unsaturated hydrocarbons is preferred. In particular, a C4 fraction containing a large amount of butadiene or a C5 fraction containing a large amount of isoprene is preferred.
The apparatus and method of the present invention are effective when applied to the case of trying to obtain a concentration of an unsaturated hydrocarbon in the petroleum fraction of normally at least 90 percent, preferably at least 95 percent (specifically, extracting and distilling the petroleum fraction to increase the concentration of the unsaturated hydrocarbon).
According to the apparatus and method of the present invention, it is possible to stably take out a target unsaturated hydrocarbon at a predetermined concentration regardless of variations in the feedstock components.